1. Field of the Invention. The present invention relates to a hygrometer and a dew-point instrument.
2. Description of the Related Art. Various hygrometers for measuring the humidity of the atmosphere are known. For example, Japanese Utility Model Registration No. 3021853 below discloses a hygrometer for measuring the humidity by using a wet and dry bulb. In this hygrometer, a dry-bulb temperature sensor and a wet-bulb temperature sensor of the same type are installed parallel to each other, a tank with water therein is placed below the wet-bulb temperature sensor, and the sensing part of the wet-bulb temperature sensor is covered with a wick made of gauze or the like impregnated with water of the tank. This hygrometer is configured to obtain a relative humidity from the difference between the dry-bulb temperature shown by the dry-bulb temperature sensor and the wet-bulb temperature shown by the wet-bulb temperature sensor.
Japanese Patent Application Publication No. 2003-194756 discloses a mirror-surface condensing dew-point instrument. This mirror-surface condensing dew-point instrument is based on the principle in which when the surface temperature of the reflecting mirror goes below the dew point of measuring gas, the optical reflectance of the surface of the reflecting mirror is reduced by dew condensation. The dew point of the measuring gas is determined by comparing the received amount of reference light that passes through the measuring gas without being reflected on the reflecting mirror, with the received amount of measuring light reflected on the reflecting mirror while passing through the measuring gas.
Specifically, in this dew-point instrument, the measuring gas is sent into the housing, and the reference light travels straight from the first light source element outside the housing to the measuring optical path inside the housing via the optical fiber, and thereafter reaches the first light receiving element outside the housing via the optical fiber again. On the other hand, the measuring light is introduced from the second light source element outside the housing into the housing via the optical fiber, is then reflected on the mirror surface of the reflecting mirror inside the housing, and thereafter reaches the second light receiving element outside the housing via the optical fiber again. The reflecting mirror has a heat pump, the cooling capacity of which is controlled by the control circuit. The reflecting mirror is cooled by this heat pump, and when the moisture within the measuring gas contacting the reflecting mirror builds up condensation on the mirror surface, the amount of light received by the second light receiving element is reduced. Then, cooling of the reflecting mirror performed by the heat pump is controlled such that the amount of light received by the second light receiving element is reduced relative to the amount of light received by the first light receiving element and such that the amount of light received by the second light receiving element becomes constant. The mirror surface temperature of the reflecting mirror is measured by the temperature detector when the amount of light received by the second light receiving element becomes constant, whereby the dew point is obtained.
However, in the hygrometer described in Japanese Utility Model Registration No. 3021853, because the force for pumping up the water is weakened when the wick becomes old and tainted, a troublesome work for replacing the wick needs to be performed each time. Therefore, there is a problem that the workload of maintaining the hygrometer increases.
The dew-point instrument described in Japanese Patent Application Publication No. 2003-194756, on the other hand, is configured by a large number of constituent parts including the two light source elements, two light receiving elements, plurality of optical fibers, reflecting mirror, heat pump, temperature detector, heat pump control circuit, temperature measuring circuit and so on. The problem therefore is that the structure of this dew-point instrument is complicated.